The present invention relates to a matrix printhead which includes a plurality of separately operable and positionable magnetic drives respectively for reciprocating print needles, wires or styli fastened to these drives.
Matrix printheads of the type to which the invention pertains are basically constructed for serial operation or as line printers. In each instance operation has as its principal goal to provide as many print dots as possible, both from the point of view of dots per unit time as well as from the point of view of overall use life. Serial matrix printers at the present time reach a performance rate of about 400 characters per second and line printers even go up to 900 characters/sec. Commensurately with these performances the matrix printers must be expected to respond to the thermal load resulting from densely clustering of the electromagnetic drives as well as to the mechanical wear resulting from high speed mechanical motions. These are important factors to be considered upon making a choice of the type and brand of printers. Aside from problems that may arise in the electric or electronic part of such a printer certain mechanical problems are to be expected. This includes for example the fastening of a print element such as a needle, wire or stylus to the respective drive. This fastening pint is so to speak a weak point within the system.
Basically two modes of fastening a print needle to its drive are known. In accordance with the first mode or type one uses the principle of transmitting movement from the magnetic drive element to the wire or stylus but not directly; rather preferred is a kinematic coupling between these elements through a force locking connection between the drive element such as a pivot armature and a physically separate print needle to which is fastened a head and it is that head which engages the particular drive element i.e. the pivot armature. The engagement should be a positive one. It has to be observed however that wire and wire head on one hand and pivot armature on the other hand have different masses. Therefore the adaptation of that mechanical system to the operating frequency of the magnetic drive is difficult; in particular, problems of parasytic oscillations and of control generally are to be expected. U.S. Pat. No. 4,478,528 is an example of this type of coupling and it is also representative of the type of structure in which the aforementioned problems arrive.
The second type or mode of coupling the first stylus or wire to its drive element avoids in principle the problems above in that the wire or styli are immediately and directly rigidly connected to some front end of the magnetic drive. This means that the needle necessarily follows the control of the drive element and its electronics. German printed patent application 2,630,931 (see U.S. patent application Ser. No. 621,526, filed Oct. 10, 1975) discloses an example of this type. However, from the point of view of manufacturing this type of drive and coupling is realizable only with difficulty. Fastening of the needle or stylus to the armature requires particular auxiliary structures whereby still the danger arises that tolerances of these assisting equipment permit only an inadequate and insufficient arrangement and orientation of the needles or wires vis-a-vis the drive element.
Another drawback of the latter known mode of fastening is to be seen in the connection proper. This connection can be done through soldering but in operation the connecting point is exposed to heat as it is developed in the drive. Heating of the needles which have been soldered, e.g. by inductive method, may in fact in extreme cases lead to fracture of the needle even after a relatively short period of operation. The cause is to be seen in microstructure changes of the material of the needles right at or near the soldering point, ether during the heating on soldering or during subsequent cooling from this soldering process. The reheating during operation will cause very soon fracture.